Now Chrysler Group plans a direct assault on Ford's claim to the high ground in fuel economy. When the re-engineered 2013 Ram 1500 arrives in showrooms this fall it will challenge Ford's F-series, which got a new engine lineup in 2011, for the title of most fuel-efficient full-sized pickup.

The mpg fixation by two of the Detroit 3 is a stark departure from past strategies for attacking the pickup market. For decades, automakers squabbled over whose truck could haul more or last longer.

General Motors, whose redesigned Chevrolet Silverado and GMC Sierra are expected to arrive in showrooms next spring, plans a more traditional line of attack. GM wants to "keep that Silverado and Sierra as workhorses," says GM North America President Mark Reuss. Unlike Ford or Chrysler, it will offer a mid-sized pickup for buyers concerned more about fuel economy than about size and muscle.

New engine options are a likely bet for Nissan's big truck. That has been the primary criticism of the modest-selling Titan since it first appeared in 2004. It has been available with only one engine, a 5.6-liter V8, and Nissan has been touting higher fuel-economy ratings on its recent model redesigns.

The Toyota Tundra currently offers two V8s and a modestly selling V6 that falls shy of the mpg numbers for Ford's V6.

"We're going to be best-in-class fuel economy in both the V6 and V8 segments," Ram brand CEO Fred Diaz told Bloomberg when the Ram 1500 was revealed this spring.

Diaz also took a shot at Ford's EcoBoost, telling The Detroit News that "turbos are very expensive to replace."

If Chrysler's talking trash, Ford executives are ready for a fight.

"We've heard what they've said. We'll see what they deliver when the EPA numbers are available," said Doug Scott, Ford's head of truck marketing. "The beauty of that is it will drive more interest and attention to the segment. We're generally on the shopping list as the segment leader."

Chevrolet will introduce a redesigned Colorado pickup in 2013.

While Ford and Chrysler gear up to fight it out for pickup mpg leadership, GM's battle strategy diverges.

Many of the urban cowboys who bought full-sized pickups five years ago have been flushed out of the market amid the tough economy and higher gasoline prices. Reuss figures the customers that remain are serious truck users--farmers and contractors who value attributes such as towing capacity over fuel economy.

For those mpg-conscious buyers who remain, Reuss says GM will have a solution that its Detroit rivals don't have: a mid-sized pickup.

A next-generation Chevy Colorado mid-sized pickup is expected in U.S. showrooms by late 2013 or early 2014. GM is expected to badge the pickup as a GMC Canyon, too, although it hasn't confirmed those plans.

In contrast, Ford has ditched its Ranger. The mid-sized Dodge Dakota died in 2011.

Raffi Festekjian, director of automotive research for J.D. Power and Associates in Westlake Village, Calif., says surveys support GM's position.

"Reliability and durability remain" the top reasons customers buy large pickups, he says. "Gas mileage is toward the bottom in terms of reasons" people buy large pickups, even though it has increased since 2006.

But Ford's Scott says fuel economy matters to buyers who use their trucks as a work tool.

Ten years ago, fuel economy "wasn't among the top 10 purchase motivations," Scott admits. The priorities were payload, durability and price. Now, though, "fuel economy is definitely firmly in that top 10 purchase reasons."

Even Ford was surprised at how quickly sales of V6 equipped F-150 pickups took off. Until 2010, Ford offered only V8s in its F-150 lineup. When Ford offered the EcoBoost V6 in February 2011, it was an immediate hit.

"We've had 13 months consecutively where the V6 mix has outstripped the V8," says Ford's Scott. The 3.5-liter V6 has accounted for 42 percent of the F-150 engine mix so far this year, while the entry-level 3.7-liter V6 accounts for an additional 12 or 13 percent, he says.

Dodge, in turn, has looked beyond a V6 for ways to boost the Ram 1500's fuel efficiency.

The 2013 Ram will be the first pickup to get a stop-start system. It will have an air suspension system that lowers the vehicle at highway speeds to improve aerodynamics, and active grille shutters on its nose to improve air flow when cooling is not required. Even the Ram's sidestep rails were extended between the front and rear wheels after engineers determined in wind-tunnel testing that doing so improved the pickup's aerodynamics.

Bob Hegbloom, director of the Ram brand, said pickup manufacturers no longer can afford to bypass technology that promises to improve fuel economy: "Anything that we can look at, I have to do."

Larry P. Vellequette contributed to this report

Pickup pecking order

Ford's F-series, the newest full-sized pickup on the block, gets fresh competition from Chevrolet and Dodge this fall. Here's how sales stacked up through May.